[O]ne person's 'barbarian' is another person's 'just doing what everybody else is doing.

No enemies had ever taken Ankh-Morpork. Well technically they had, quite often; the city welcomed free-spending barbarian invaders, but somehow the puzzled raiders found, after a few days, that they didn't own their horses any more, and within a couple of months they were just another minority group with its own graffiti and food shops.

The Barbarian hopes — and that is the mark of him, that he can have his cake and eat it too.He will consume what civilization has slowly produced after generations of selection and effort, but he will not be at pains to replace such goods, nor indeed has he a comprehension of the virtue that has brought them into being. Discipline seems to him irrational, on which account he is ever marvelling that civilization, should have offended him with priests and soldiers.... In a word, the Barbarian is discoverable everywhere in this, that he cannot make: that he can befog and destroy but that he cannot sustain; and of every Barbarian in the decline or peril of every civilization exactly that has been true.We sit by and watch the barbarian. We tolerate him in the long stretches of peace, we are not afraid. We are tickled by his irreverence; his comic inversion of our old certitudes and our fixed creed refreshes us; we laugh. But as we laugh we are watched by large and awful faces from beyond, and on these faces there are no smiles.

[H]e is a barbarian, and thinks that the customs of his tribe and island are the laws of nature.

There is always a city. There is always a civilisation. There is always a barbarian with a pickaxe. Sometimes you are the city, sometimes you are the civilisation, but to become that city, that civilisation, you once took a pickaxe and destroyed what you hated, and what you hated was what you did not understand.